The invention relates to a method of coating the inside of a tube of electrically insulating material with a layer of an electrically insulating material. The coating is provided by reactive deposition of a layer of an electrically insulating material from a gas mixture which is passed through the tube. The tube is heated thermally, and the deposition is activated by a plasma which is produced by microwaves and is reciprocated in the tube.
The invention further relates to a device for coating the inside of a tube of electrically insulating material with a layer of an electrically insulating material. The device comprises a gas inlet system for supplying the gas mixture to the tube, a microwave generator and a microwave resonator to produce and maintain a plasma in the gas mixture in the tube, means to reciprocate the plasma in the longitudinal direction of the tube, and means to thermally heat the tube.
A method and a device of this type are described for example, in articles by P. Geittner, et al. entitled "Low-loss optical fibers prepared by plasma-activated chemical vapor deposition (CVD)" (Applied Physics Letters, Vol. 28, No. 11, pages 645-646, June 1, 1976) and D. Kuppers, et al. entitled "Preparation of Optical Waveguides with the Aid of Plasma-Activated Chemical Vapour Deposition at Low Pressures" (Topics in Current Chemistry, Vol. 89, pages 107-131, 1980). In these articles, the manufacture of optical fibers according to the PCVD method is described. In this method a layer of a light-conducting material is deposited as electrically insulating material. The material is deposited by means of a plasma from a gas mixture containing the volatile starting materials. The material is deposited on the inside of the tube.
The tube either consists of synthetically manufactured amorphous quartz (fused silica, quartz glass) or of amorphous quartz manufactured from quartz crystals by melting. The tube may optionally be doped. It might alternatively consist of both amorphous quartz manufactured synthetically and of amorphous quartz manufactured from quartz crystals by melting.
After a quantity of light-conducting material corresponding to the intended optical fiber construction has been deposited, the tube is collapsed so as to form a solid preform from which optical fibers are drawn.
As is described in greater detail in the abovementioned articles, tube wall temperatures of between 1100.degree. and 1300.degree. C. are required for the deposition of adhering layers of a light-conductive material. In order to ensure that the tube reaches these temperatures during coating, a furnace is used as an additional energy source. Either a mobile furnace is provided, which is moved synchronously with the microwave resonator along the tube, or a stationary furnace is provided, in which a cooled microwave resonator is moved along the tube.
Tubes from which qualitatively high-grade optical fibers may be made can be produced by using both types of furnaces. However, certain disadvantages are also associated with both furnace types. Disadvantages of these furnaces are, for example, their cumbersome and expensive constructions, their large spaces occupied, their high inert masses, their high thermal inertia, and their high energy consumptions. Particularly disturbing is that most of the process heat provided by the plasma is lost by cooling the resonator and by radiation. Simultaneously, however, parts of the tube not screened off by the moving resonator must be additionally heated.